Ultrasonic (US) pen systems generally include a pen and a panel. A user may use the pen to write on the panel. The pen may generate ultrasonic signals which the panel digitizes and provides to a computer. Thus, the signals transmitted by the pen may be utilized in determining the location of the pen.
Some ultrasonic pen systems may utilize an algorithm based on time of arrival (TOA) of the ultrasonic signals to determine pen locations. For example, one TOA-based algorithm may utilize the characteristics of the ultrasonic signals to determine pen locations. However, since ultrasonic signals for pens from different manufacturers may have different characteristics, these pens may not be compatible with panels provided by other manufacturers. Also, another TOA-based algorithm may utilize infrared (IR) synchronization signals to determine pen locations. Accordingly, such pen systems utilize IR transmitters and receivers which increase the expense of such systems. Also, the IR signals may be prone to environmental interference such as sunlight, temperature, artificial light, and the like. Some systems may also be prone to acoustic or mechanical noise such as sonic or ultrasonic noise and ultrasonic reflection.
Other techniques that may be utilized in locating mobile units may lack sufficient resolution for pen system applications. For example, a global positioning system (GPS) may provide sufficient resolution to locate a cellular phone on a street but lacks the resolution to recognize pen locations on a panel. Also, angle of arrival (AOA) algorithms suffer from blind regions and may utilize expensive antenna arrays that may be impractical for ultrasonic pen systems.